In this proposal I describe an approach to study x-ray induced mutations in transgenic mice. The approach is based on an integrated lambda phage shuttle vector which carries target genes in which mutants can be scored and analyzed. This vector is amenable to study in vivo in transgenic mice as well as in cell culture systems. To produce transgenic mice, mouse oocytes are injected with lambda phage DNA containing the supF gene of E. coli and the c1857 gene of lambda as target genes for mutagenesis. Rescue of the lambda vector DNA for analysis is accomplished by addition of the transgenic mouse DNA (containing the integrated lambda DNA) to lambda in vitro packaging extracts which can identify, cut out, and package the lambda DNA, from within the mouse DNA, into viable phage particles. The availability of whole animals with recoverable shuttle vectors with target genes makes possible the study of physiologically relevant parameters in exposure to ionizing radiation. Some of these factors include dose-rate effects, tissue and organ specific differences, age-related effects, and tumor versus normal tissue differences. These factors have not yet been examined in vivo because no experimental system has been yet developed to evaluate them.